Electronic article surveillance (EAS) systems are known in which a transmitter radiates a pulsed magnetic field into a surveillance area. When an article having an attached system tag is positioned within the surveillance area, the radiated magnetic field excites the tag, causing the tag to generate a detectable response signal. A receiver, which is enabled while the pulsed radiated magnetic field is turned off, detects the tag's response signal and initiates an appropriate action.
In a typical EAS transmitter the pulsed magnetic field is generated by periodically driving an antenna with a square wave having a frequency which defines a desired operating frequency for the system. The square wave comprises the output voltage of a switching power amplifier, and the antenna has resistive-inductive-capacitive (R-L-C) components that are appropriately selected to permit the antenna to resonate at the desired operating frequency. This maximizes the current flowing through the antenna which, in turn, maximizes the amplitude of the radiated magnetic field.
In order to permit currents on the order of a few amps to flow through the antenna, the values of the R-L-C components must be selected so that the antenna has a very large Q factor. To obtain such a large Q, tight control of the absolute value of the R-L-C components is required. For example, in an antenna where the inductance L and resistance R are fixed values, governed by the geometry of the antenna coils, the total capacitance C must be accurate to within a 1% to 2% tolerance. This is usually achieved using precision capacitors or a trimming capacitor, thereby adding cost to the production of the transmitter. Further, an antenna having such a high Q limits the radiated magnetic field to a very narrow range of frequencies surrounding the resonant frequency of the antenna, thereby preventing multifrequency operation of the system.
A further problem with a transmitter of the above-type is that the inductance value L changes based upon external conditions. For example, moving a large metal object into the radiated magnetic field has the effect of changing the inductance value L of the antenna. This change in inductance value shifts the resonant frequency and Q of the antenna which causes the current flowing through the antenna to be reduced. The reduced current flow decreases the magnitude of the radiated magnetic field and, therefore, reduces the effectiveness of the EAS system.
Another disadvantage of the known EAS transmitter is that the R-L-C components of the antenna form a two pole filter which provides only a -6 db/octave filtering of the transmitter drive signal. This magnitude of filtering may not reduce the harmonic content of the radiated magnetic field to a level sufficient to meet all regulatory requirements. Adding additional R-L-C components to the antenna is one technique which provides additional filtering. However, the added R-L-C components create problems of power dissipation, as the components must carry the large currents which are passed through the antenna. Further, the introduction of additional R-L-C components greatly increases the rise and fall times of the envelope of the radiated magnetic field when the field is cycled on and off.
It is also desired in the known EAS transmitters to be able to control the rise and fall times of the envelope of the radiated magnetic field. This control must be effected in a manner to satisfy competing requirements. One requirement necessitates a relatively short fall time for the envelope so that the falling edge of the envelope does not interfere with the receipt of the maximum tag response occurring immediately after the transmitter is turned off. A second requirement necessitates a longer fall time, as well as a relatively long rise time, so as to limit the harmonics usually generated by sharp edges, i.e., rapid rise and fall times, of the envelope. In present EAS transmitters, there is no mechanism for optimally satisfying these competing requirements.
It is therefore an object of the present invention to provide a transmitter for an EAS system which forms a pulsed radiated magnetic field which is not adversely effected by changes in the characteristics of the antenna;
It is a further object of the present invention to provide a transmitter for an EAS system which forms a pulsed radiated magnetic field having a reduced harmonic content; and
It is yet a further object of the present invention to provide a transmitter for an EAS system which forms a pulsed radiated magnetic field having an envelope with predetermined rise and fall time characteristics.